Fix StackOverflow on ToString method

Issue found while fixing instrumentation tests:
`com.android.bluetooth.pbap.BluetoothPbapObexServerTest#testLogHeader`

Test: BluetoothInstrumentationTests
Change-Id: I98e5a2ad1d9ae3fa287ea739ff35cb75fb139c7e
diff --git a/src/com/android/obex/HeaderSet.java b/src/com/android/obex/HeaderSet.java
index 15c128d..d1afe4f 100644
--- a/src/com/android/obex/HeaderSet.java
+++ b/src/com/android/obex/HeaderSet.java
@@ -729,7 +729,7 @@
 
     @Override
     public String toString() {
-        return "HeaderSet " + this + ": NAME=" + mName;
+        return "HeaderSet " + System.identityHashCode(this) + ": NAME=" + mName;
     }
 
     public String dump() {